1. Field of Invention
The invention relates generally to the color grading of gemstones such as rubies, emeralds and sapphires, and in particular to classified color guides for gems, each guide facilitating the color grading of stones falling within a specified class by providing a series of simulated reference standards against which an observer can compare an unknown in that class to determine its color grade.
In establishing the economic value of a precious or semiprecious stone, color represents the most influential factor. Before addressing the problem of color grading gemstones, the essential nature of color must first be examined.
Within the broad spectrum of radiant wave energy lies a narrow band of visible energy which extends through a wavelength range of about 380 to 760 nanometers. Although wavelengths lying above or below this band can affect a gem's appearance, such as those in the infrared or ultraviolet region, they do not directly stimulate the receptors of the eye and are therefore invisible.
A light source emitting radiant energy that is relatively balanced in all visible wavelengths appears to the eye as white. Without light, there can be no color, for color simply represents an imbalance of visible energy reaching the eye from a light source or an object, an imbalance being any deviation from the average amount of all wavelengths in the visible band. All physical objects when exposed to light more or less absorb, transmit or reflect the light impinging thereon and therefore have a modifying effect--reducing both the amount of energy and the nature of the light waves which reach the eye. Thus an object such as a gem affords a particular color impression because it reflects or transmits light waves only in certain narrow ranges, while absorbing all others.
In interpreting the color of an object such as a colored stone, several variables come in play. Not only is color judgment influenced by the nature and intensity of the light source employed, but also by the method of viewing the illuminated object and by its physical configuration and degree of transparency. Moreover, one must take into account the subjective sensitivity of the observer to color.
Thus if one examines a Thai ruby under an incandescent light bulb, the predominance of red wavelengths in this source will accentuate the essential redness of the stone. But when using "daylight" fluorescent illumination to examine the same stone, this will generally cause the stone to assume a purplish-red appearance because of dominant blue wavelengths in this source. On the other hand, under natural daylight illumination of the same ruby, an orange or brownish coloration will often become apparent. In each instance, therefore, the absorption and transmission characteristics of the stone will interact with whatever examination light is available to alter the visible color composition.
A fluorescent "daylight" fixture is a useful illumination source for gem examination; for while this source does not exactly duplicate the unique spectral energy distribution of diffused light reflected from the North Sky, it has the advantage of producing a constant and uniform lighting environment and is therefore an acceptable light source for color grading gems by means of the color guides in accordance with the invention to be described hereinafter.
The color content of a transparent stone may be identified in terms of three basic variables; namely, hue, tone and intensity. Hue is the visible sensation imparted to an observer that permits him to distinguish one color from another. Thus the visual distinctions existing between red, blue, green and other colors represent the hue of a stone. Hue also encompasses color gradations such as orangy-red, yellowish-green and greenish-blue.
Tone is the visual sensation that makes it possible for an observer to recognize shadings in a particular hue on a scale running from light to dark. Thus adding increasing amounts of white to a particular hue, say, pure green, will dilute this color into a range of progressively lighter tints of green. If, however, one adds to pure green increasing amounts of black, this will create progressively darker shades of this color.
Intensity is the visual sensation that reflects the degree of vividness or richness in a color on a scale ranging from vivid to dull. Thus in gem materials, adding brown or gray to a vivid color will reduce its intensity and render it dull to an extent depending on the relative amount of the additive. Conversely, the absence of brown or gray in a gem generally contributes to the vividness of the gem's body color.
When judging the value of a gemstone by grading its color, one is not dealing with a flat, two-dimensional surface, but with a three-dimensional, multi-faceted body cut to a variety of shapes and facet configurations. Color appraisal is further complicated by the presence of pleochroism in many gem bodies. This dichroic or trichroic property describes a mineral's ability to directionally exhibit two or more colors when viewed in various directions or in the same direction. Traditionally, only diamonds, emeralds, rubies and sapphires were considered to be "precious" minerals; all the rest being treated as "semiprecious." However, these terms are currently regarded as meaningless in view of the wide quality variations encountered in any specific gemstone where beauty is the most important desideratum and rarity is also a significant factor.
Transparent gemstones are usually cut into prism-like or faceted configurations to exploit repeated light reflections which create a sparkling, brilliant effect and color amplification typical of fashioned gemstones. Faceted gems are generally cut either in a "brilliant" or a "step-cut" style. The standard brilliant cut is basically composed of numerous triangular facets completely covering the bottom of the stone and partially covering the top thereof in a band directly above the girdle, the major portion of the top being truncated by a single large table facet. The step-cut consists of a series of rectangular or trapezoidally-shaped facets or steps covering the bottom and again partly covering the top. A combination of these two basic cutting styles is commonly used to create various optical effects including scintillation, dispersion and color amplification.
In all faceted gems, light rays enter the top mainly through the table facet, the rays then striking the inclined bottom facets where they are internally reflected upwardly toward the observer's eye. The brilliant cut is largely reserved for diamonds, whereas the step cut or a combination of step and brilliant cuts, is favored for all other gemstones, such as emeralds, rubies and sapphires.
One commonly used technique for grading a colored stone makes use of a set of standards that function as physical reference points for comparing unknowns therewith in a standardized light environment by a trained observer. In the gem industry, these reference standards often take the form of actual stones to which a fixed or general grade has been assigned. Thus for the color-grading of emeralds, one may use a set of, say, ten actual emeralds of progressively better color grade, a match then being sought by the observer between the unknown sample and one of these actual standards.
One disadvantage of using real stones as standards is that they are subject to wear and tear, and may in time require recutting, this giving rise to a change in appearance of the stone. Then, of course, one is faced with the cost of providing a set of actual reference standards for each type of gemstone. The investment entailed thereby may be prohibitive for the typical appraiser.
To overcome this problem, use may be made of glass or plastic imitations which in some cases nearly duplicate the appearance of actual gems. But their color range is limited and difficult to control. This introduces a margin of error in color appraisal which may render it unacceptable for purposes of certification. Moreover, the accurate duplication and reproducibility of actual gem master sets or gem simulants for communication between appraiser and/or dealer in colored stones is a nearly impossible task.
Another approach is to use visual colorimeters in which a reference standard is produced by mixing adjustable amounts of red, green and blue light in order to match the color of an unknown sample. In practice, this leaves much to be desired; for a three-dimensional, multi-faceted gem affords a complex color impression that cannot be accurately simulated by visual colorimetry.
Nor is electronic colorimetry entirely effective in the color grading of gems. In this approach, color filters and sensors are used as an alternative to the eye to electronically monitor the quality and quantity of light emerging from an unknown, the various color components being electronically sensed and measured. But here again, the complicated nature of the gem structure being analyzed creates difficulties in interpretation that render standard measurement techniques of limited value.
One may also analyze the color of a gemstone by spectrophotometry, making use of an instrument which breaks light into its component wavelengths, each slice in the spectrum passing through the sample being tested, and being then converted into a corresponding digital value which is analyzed in a computer. Though this approach is at least theoretically capable of affording a precise analysis of the color characteristics of a gem stone, the cost of this instrument and the operating skills entailed thereby are beyond the means and capabilities of the average stone appraiser.